Well, he's not going to be able to run XP unless he has 1gb of memory, and if he has a PC with 1gb of memory I am sure he has more than a 300-500Mhz PC, as those type machines were never sold with 1gb ram.
Poor logic. As already stated, XP was installed on computers with as little as 128mb of RAM, and I've personally installed it on computers with 64mb... NOT something you want to do if you have any other option. I've seen Vista on machines with 512mb of RAM; not a pretty sight.
With that said, a decent Pentium III with around 1gb of RAM should run newer versions of DOSBox alright. Daggerfall is playable on my Aspire One (1.6ghz Atom, 1.5gb RAM), which is much slower than a comparable Pentium III.
Post your system specs so someone can guide you in a better direction.